BISMUTH-BEARING ASSEMBLAGES FROM THE NORTHERN PENNINE OREFIELD

Bismuthinite-bearing quartz veins from the Alston Block of the North P ennine Orefield are all close to, or above, the Rookhope and Tynehead cupolas of the buried Weardale Granite. They are uniform in compositio n and paragenesis and are earlier than the main fluorite-baryte-galena -sphalerite mineralization of the orefield. Rhythmical crystallization of quartz, chalcopyrite and minor pyrite is followed by fluorite-quar tz-chalcopyrite-minor sphalerite-altered pyrrhotite mineralization. Ea rly tin-bearing (up to 0.29 wt.% Sn) chalcopyrite encloses trace amoun ts of bismuthinite (Bi2S3), synchysite (CaREE(CO3)F-2), argentopentlan dite (Ag(FeNi)(8)S-8) (close to being stoichiometric), pyrrhotite, cub anite and cosalite (Pb2Bi2S5), while early pyrite carries monoclinic p yrrhotite (close to Fe7S8) and tungsten-bearing cassiterite (up to 1.0 3 wt.% WO3). Bismuthinite is macroscopically visible and is associated with native bismuth and small, fine-grained, spherical aggregates tha t qualitative analysis suggests may be cosalite crystals. Synchysite a nd more rarely monazite, xenotime and adularia are intergrown with bis muthinite. These mineralogical data form part of the basis for an incr easing awareness of the contribution of the Weardale Granite to the ea rly phases of mineralization in the Alston Block.